fix(gateway): 修复仅配置 provider retry 时 429 不重试

[Kayphoon]

Summary

• attempt.rs: 从 failover_rules/endpoint/provider 三级读取 max_retries 生成本地 attempt slots，支持配置 provider retry 后 429 自动重试，限制最大 99 并兼容 legacy 默认值 2

Verification

• cargo fmt --manifest-path apps/aether-gateway/Cargo.toml -- --check
• cargo test --manifest-path apps/aether-gateway/Cargo.toml build_local_attempt_identities -- --nocapture
• cargo test --manifest-path apps/aether-gateway/Cargo.toml

[Kayphoon]

Superseded by clean feature-branch PR #337 to remove the accidental merge commit from the head branch.